1. Field of the Invention
This invention relates to semiconductor device packages and more specifically, relates to wirebonded semiconductor device packages having semiconductor devices that include elongated electrodes.
2. Description of the Art
III-nitride based power semiconductor devices, such as Gallium Nitride (GaN) based devices, are desirable for power applications. Example III-nitride based devices include diodes, unidirectional switches, and bi-directional switches. These devices are lateral conductive devices with the power electrodes and control electrodes disposed along a top surface of the devices and with the bottom surface of the devices being electrically non-conductive.
For example, referring to FIG. 1A there is shown a top plan view of a III-nitride unidirectional switching device 10. Device 10 includes a drain electrode 22 and a source electrode 23, which are arranged as interdigitated portions. As shown, each of these electrodes also includes an elongated runner 21 or 24 that extends substantially along the length of the device along respective edges. Runner 21 is connected to the interdigitated portions of drain electrode 22 and runner 24 is connected to the interdigitated portions of source electrode 23. Wirebonds may be formed to the runners to interface with the electrodes. Device 10 also includes a gate electrode 20 formed in a serpentine shape between drain electrode 22 and source electrode 23.
Similarly, referring to FIG. 1B there is shown a top plan view of a III-nitride bi-directional switching device 30. Device 30 may have a form as disclosed in U.S. Publication No. U.S. 2005-0189561 (U.S. patent application. Ser. No. 11/056,062), entitled “III-Nitride Bidirectional Switch,” by Daniel M. Kinzer et al., and assigned to the assignee of the present application. The contents of U.S. Publication No. U.S. 2005-0189561 are hereby incorporated by reference as if fully set forth in its entirety herein.
Device 30 includes first and second power electrodes 25 and 26, which are arranged as interdigitated portions. Again, each power electrode includes an elongated runner 42 or 40 that extends substantially along the length of the device, with runner 42 being connected to respective portions of power electrode 25 and runner 40 being connected to respective portions of power electrode 26. Similar to above, wirebonds may be formed to these runners to interface with the power electrodes. Device 30 also includes first and second gate electrodes 32 and 34, with portions of each electrode being disposed between the power electrodes. Elongated runners 46 and 48 are also provided, with runner 46 electrically connecting the portions of gate electrode 32 and runner 48 electrically connecting the portions of gate electrode 34. By controlling each gate electrode, the switch may conduct current in either a forward direction or a reverse direction. Accordingly, either power electrode may serve as the drain or source electrode.
Similarly, for a III-nitride based diode, the device may only include power electrodes.
In general, it is desirable to adapt conventional device package formats, such as TO-220 and SIP package formats, to house III-nitride based semiconductor devices. For example, referring to FIG. 2 there is shown a TO-220 device package 100 of the prior art, this package having a III-nitride based device 101 disposed therein. In this example, device 101 is a III-nitride based unidirectional switching device having a source electrode 23, drain electrode 22, and a gate electrode 20. Device package 100 also includes a lead frame 102 that has a die pad 104 and a plurality of terminal leads, such as leads 110-118. As shown in FIG. 2, the respective bond pads 110a-118a of these terminal leads are typically arranged so as to lie across the bottom of the device package adjacent to the lower edge of die pad 104. Device package 100 also includes a protective housing 105 that covers device 101 and at least a portion of the top surface of lead frame 102.
As shown in FIG. 2, the electrically non-conductive bottom surface of device 101 is mounted to die pad 104 using an adhesive such as solder, epoxy adhesive, or the like, thereby exposing the electrodes along the top surface of the device. Source electrode 23 of device 101 may be electrically connected to terminal lead 112 through a plurality of wirebonds 120 that extend from the runner of the electrode to bond pad 112a. Similarly, drain electrode 22 of device 101 may be electrically connected to terminal lead 118 through a plurality of wirebonds 121 that extend from the runner of the electrode to bond pad 118a. Gate electrode 20 of the device may be electrically connected through wirebond 122 to bond pad 112a of terminal lead 112.
Significantly and as shown in FIG. 2, because the runners of the power electrodes of III-nitride based devices are elongated and because the bond pads of the terminal leads of conventional device packages are typically formed along the bottom of the package, the plurality of wirebonds between respective power electrodes and bond pads are long and of varying/unequal lengths, thereby making the device packages unsuitable. For example, as shown in FIG. 2, the plurality of wirebonds 120 of source electrode 23 and the plurality of wirebonds 121 of drain electrode 22 are long and of varying/unequal lengths. Notably, the long length of the wirebonds increases package resistance, adds unwanted inductance, and also increases cost. The length of the wirebonds also makes the wirebonds susceptible to wire sweep during the mold process of forming protective housing 105. In addition, the unequal lengths of the respective wirebonds of given electrode results in the wirebonds having different resistances, which causes unequal current distribution across the wirebonds. Significantly, the varying lengths of the wirebonds also result in unequal current distribution in the III-nitride based device.
It is noted that similar problems like those described above also occur when packaging silicon based devices that include one or more elongated electrodes.